In electronic devices such as a mobile phone, a smart phone, a tablet terminal and a digital camera, multilayer flexible printed circuit boards are used along with downsizing, lightening and higher performance. Higher performance and higher functionality of such electronic devices have been dramatically increasing a data amount handled by the electronic devices in recent years. Moreover, a processing speed of signals in the electronic devices has been further improved as a result of shifting from parallel wiring to serial wiring in order to reduce the area of a substrate. Accordingly, a transmission speed of signals passing through the multilayer flexible printed circuit board has also increased.
A microstrip line and a strip line are conventionally known as transmission paths of high-speed signals. In particular, since the strip line has a structure in which ground layers above and below the same interpose a signal line, it has an advantage of a high shielding effect of electromagnetic waves emitted from the signal line and external electromagnetic waves. Moreover, the strip line can be relatively easily formed in the multilayer flexible printed circuit board.
Patent Literature 1 discloses a multilayer flexible printed circuit board having a strip line. In this multilayer flexible printed circuit board, ground layers of the strip line are constituted of copper foils of a copper clad laminated sheet.